The following code is quite trivial and I expected that it should compile fine.
struct A
{
struct B
{
int i = 0;
};
B b;
A(const B& _b = B())
: b(_b)
{}
};
I've tested this code with g++ version 4.7.2, 4.8.1, clang++ 3.2 and 3.3. Apart from fact that g++ 4.7.2 segfaults on this code (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57770), the other tested compilers give error messages that don't explain much.
g++ 4.8.1:
test.cpp: In constructor ‘constexpr A::B::B()’:
test.cpp:3:12: error: constructor required before non-static data member for ‘A::B::i’ has been parsed
struct B
^
test.cpp: At global scope:
test.cpp:11:23: note: synthesized method ‘constexpr A::B::B()’ first required here
A(const B& _b = B())
^
clang++ 3.2 and 3.3:
test.cpp:11:21: error: defaulted default constructor of 'B' cannot be used by non-static data member initializer which appears before end of class definition
A(const B& _b = B())
^
Making this code compilable is possible and seems like it should make no difference. There are two options:
struct B
{
int i = 0;
B(){} // using B()=default; works only for clang++
};
or
struct B
{
int i;
B() : i(0) {} // classic c++98 initialization
};
Is this code really incorrect or are the compilers wrong?
Best Answer
Well, neither. The standard has a defect -- it says both that
A
is considered complete while parsing the initializer forB::i
, and thatB::B()
(which uses the initializer forB::i
) can be used within the definition ofA
. That's clearly cyclic. Consider this:This has a contradiction:
B::B()
is implicitlynoexcept
iffA()
does not throw, andA()
does not throw iffB::B()
is notnoexcept
. There are a number of other cycles and contradictions in this area.This is tracked by core issues 1360 and 1397. Note in particular this note in core issue 1397:
That's a special case of the rule that I implemented in Clang to resolve this issue. Clang's rule is that a defaulted default constructor for a class cannot be used before the non-static data member initializers for that class are parsed. Hence Clang issues a diagnostic here:
... because Clang parses default arguments before it parses default initializers, and this default argument would require
B
's default initializers to have already been parsed (in order to implicitly defineB::B()
).